What Is The Cosmic Microwave Background Radiation?

What Is The Cosmic Microwave Background Radiation?

The Cosmic Microwave Background Radiation is the afterglow of the Big Bang; one of the strongest lines of evidence we have that this event happened. UCLA’s Dr. Ned Wright explains.

“Ok, I’m Ned Wright, and I’m a professor of physics and astronomy at UCLA, and I work on infrared astronomy and cosmology.”

How useful is the cosmic microwave background radiation?

“Well, the most important information we get is from the cosmic microwave background radiation come from, at the lowest level, is it’s existence. When I started in astronomy, it wasn’t 100 percent certain that the Big Bang model was correct. And so with the prediction of a cosmic microwave background from the Big Bang and the prediction of no cosmic microwave background from the competing theory, the steady state, that was a very important step in our knowledge.”

“And then the second aspect of the cosmic microwave background that is very important, is that it’s spectrum is extremely similar to a black body. And so, by being a black body means that universe relatively smoothly transitioned from being opaque to being transparent, and then we actually see effectively an isothermal cavity when we look out, so it looks very close to a black body.”

“And the fact that we are moving through the universe can be measured very precisely by looking at what is called the dipole anisotropy of the microwave background. So one side of the sky is slightly hotter (about 3 millikelvin hotter) and one side of the sky – the opposite side of the sky – is slightly colder (about 3 millikelvin colder), so that means that we are moving at approximately a tenth of a percent of the speed of light. And in fact we now know very precisely what that value is – it’s about 370 kilometers per second. So that’s our motion, the solar system’s motion, through the universe.”

“An then the final piece of information we’re getting from the microwave background now, in fact the Planck satellite just gave us more information along these lines is measurement of the statistical pattern of the very small what I call anisotropies or little bumps and valleys in the temperature. So in addition to the 3 millikelvin difference, we actually have plus or minus 100 microkelvin difference in the temperature from different spots. And so, when you look at these spots, and look at their detailed pattern, you can actually see a very prominent feature, which is there’s about a one and a half degree preferred scale, and that’s what’s caused by the acoustic
waves that are set up by the density perturbations early in the history of the universe, and how far they could travel before the universe became transparent. And that’s a very strong indicator about the universe.”

What does it tell us about dark energy?

“The cosmic microwave background actually has this pattern on a half degree scale, and that gives you effectively a line of position, as you have with celestial navigation where you get a measurement of one star with a sextant, then you get a line on the map where you are. But you can look at the same pattern – the acoustic wave setup in the universe, and you see that in the galaxy’s distribution a lot more locally. We’re talking about galaxies, so it might be a billion light years away, but to cosmologists, that’s local. And these galaxies also show the same wave-like pattern, and you can measure that angle at scale locally and compare it to what you see in history and that gives you the crossing line of position. And that really tells us where we are in the universe, and how much stuff there is and it tells us that we have this dark energy which nobody really understands what it is, but we know what it’s doing. It’s making the universe accelerate in it’s expansion.”

ALMA Warms Up the View of the Coldest Place In the Universe

Where is the coldest place in the Universe? Right now, astronomers consider the “Boomerang Nebula” to have the honors. Located about 5,000 light-years away in the constellation Centaurus, this pre-planetary nebula carries a temperature of about one Kelvin – or a brisk, minus 458 degrees Fahrenheit. That makes it even colder than the natural background temperature of space! What makes it more frigid than the elusive afterglow of the Big Bang? Astronomers are employing the powers of the Atacama Large Millimeter/submillimeter Array (ALMA) telescope to tell us more about its chilly properties and unusual shape.

The “Boomerang” is different all the way around. It is not yet a planetary nebula. The fueling light source – the central star – just isn’t hot enough yet to emit the massive amounts of ultra-violet radiation which lights up the structure. Right now it is illuminated by starlight shining off its surrounding dust grains. When it was first observed in optical light by our terrestrial telescopes, the nebula appeared to be shifted to one side and that’s how it got its fanciful name. Subsequent observations with the Hubble Space Telescope revealed an hour-glass structure. Now, enter ALMA. With these new observations, we can see the Hubble images only show part of what’s happening and the dual lobes seen in the older data were probably only a “trick of the light” as presented by optical wavelengths.

“This ultra-cold object is extremely intriguing and we’re learning much more about its true nature with ALMA,” said Raghvendra Sahai, a researcher and principal scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of a paper published in the Astrophysical Journal. “What seemed like a double lobe, or ‘boomerang’ shape, from Earth-based optical telescopes, is actually a much broader structure that is expanding rapidly into space.”

So what is going on out there that makes the Boomerang such a cool customer? It’s the outflow, baby. The central star is expanding at a frenzied pace and it is lowering its own temperature in the process. A prime example of this is an air conditioner. It uses expanding gas to create a colder core and as the breeze blows over it – or in this case, the expanding shell – the environment around it is cooled. Astronomers were able to determine just how cool the gas in the nebula is by noting how it absorbed the constant of the cosmic microwave background radiation: a perfect 2.8 degrees Kelvin (minus 455 degrees Fahrenheit).

Credit: NASA/ESA
Credit: NASA/ESA
“When astronomers looked at this object in 2003 with Hubble, they saw a very classic ‘hourglass’ shape,” commented Sahai. “Many planetary nebulae have this same double-lobe appearance, which is the result of streams of high-speed gas being jettisoned from the star. The jets then excavate holes in a surrounding cloud of gas that was ejected by the star even earlier in its lifetime as a red giant.”

However, the single-dish millimeter wavelength telescopes didn’t see things the same as Hubble. Rather than a skinny waist, they found a fuller figure – a “nearly spherical outflow of material”. According to the news release, ALMA’s unprecedented resolution permitted researchers to determine why there was such a difference in overall appearance. The dual-lobe structure was evident when they focused on the distribution of carbon monoxide molecules as seen at millimeter wavelengths, but only toward the inside of the nebula. The outside was a different story, though. ALMA revealed a stretched, cold gas cloud that was relatively rounded. What’s more, the researchers also pinpointed a thick corridor of millimeter-sized dust grains enveloping the progenitor star – the reason the outer cloud took on the appearance of a bowtie in visible light! These dust grains shielded a portion of the star’s light, allowing just a glimpse in optical wavelengths coming from opposite ends of the cloud.

“This is important for the understanding of how stars die and become planetary nebulae,” said Sahai. “Using ALMA, we were quite literally and figuratively able to shed new light on the death throes of a Sun-like star.”

There’s even more to these new findings. Even though the perimeter of the nebula is beginning to warm up, it’s still just a bit colder than the cosmic microwave background. What could be responsible? Just ask Einstein. He called it the “photoelectric effect”.

Original Story Source: NRAO News Release.

Big Bang Timeline

A fraction of a second after the big bang, the universe underwent inflation - but what does that mean? credit: NASA/WMAP
Time line of the Universe (Credit: NASA/WMAP Science Team)

The Big Bang timeline is basically just a list of relative times at which the major events in the history of the universe occurred, per the collection of theories, models, and hypotheses which together form what is called the Big Bang theory.

The start – when time began, when t = 0 – is not actually part of the Big Bang timeline (!), contrary to popular belief. That’s because the two theories of physics which are at the heart of the Big Bang theory – General Relativity (GR) and the Standard Model (of particle physics; SM for short) – are mutually incompatible, and that incompatibility becomes so intolerable that saying anything about what happened in the first Planck second (approx 10-43 second) is meaningless.

In fact, the closer to the Planck regime – when GR and the SM are utterly incompatible – the less reliable are our descriptions … but the relative times are nonetheless pretty good.

Actually, that’s not quite true … what is relatively certain are temperatures; forces, matter, and radiation interact in very distinct ways, depending on the temperature (and pressure, or density), but converting from temperature back to time depends on various parameters which are not so well pinned down. However, once the average mass-energy density of the universe, today, is estimated, the clock can be wound back with some confidence (it’s ~six hydrogen atoms per cubic meter, or about 7 x 10-27 kg/m3).

Around 10-35 seconds leptons and baryons were created (the strong force became a distinct force), and inflation caused the universe to expand so much that the part which later became our observable universe was both flat (no curvature, in the GR sense) and incredibly smooth (with only tiny variations in density due to quantum effects).

At around 10-11 seconds the electromagnetic and weak force became distinct.

And by about a microsecond the universe underwent another phase change … it was no longer a quark-gluon plasma, but hadrons formed (protons and neutrons).

When t = 1 second (more or less), nuclear reactions produced light nuclides, such as deuterium and helium-3 (before this time the universe was too hot for them to form) – Big Bang nucleosynthesis.

The earliest part of the universe we can still see, directly, happened when the electrons and protons (and other nuclei) combined to form hydrogen atoms; this is the recombination era, and we see it today as the cosmic microwave background … and gravity took over as the dominant force (before this it was electromagnetism – the universe was ‘radiation dominated’ – and before that, at the time of nucleosynthesis, the strong and weak forces ruled).

The rest, as they say, is history … the Dark Ages (during which the first stars were formed), the era of recombination (when stars and quasars ionized the diffuse hydrogen), galaxy formation, … and then about 13.4 billion years later we observed the skies and worked out the timeline!

There’s a lot of good material on the web on the Big Bang timeline; here are some: John Baez (who’s always worth reading) has a brief timeline, in terms of temperature; there’s a more extensive one from the University of Wisconsin-Madison, and perhaps the best, A Brief History of the Universe (University of Cambridge).

Want to explore more? Here are some of the many Universe Today articles on the Big Bang timeline: Cosmologists Look Back to Cosmic Dawn, A Star as Old as the Universe, and Book Review: The Mystery of the Mission Antimatter.

Astronomy Cast has several episodes for you to explore, to learn more about the Big Bang timeline; here are a few: The Big Bang and Cosmic Microwave Background, Inflation, and this 2009 Questions Show.


Microwave Radiation

In the microwave in your kitchen, food gets cooked (or heated) by absorbing microwave radiation, which is electromagnetic radiation between the (far) infrared and the radio, in the electromagnetic spectrum. The microwave region is rather broad, and somewhat vague, because the overlap with the radio (at around 1 meter, or 300 MHz) is not clear-cut, nor is the overlap with the sub-millimeter (or terahertz) region (at around 1 mm, or 300 GHz).

In astronomy, by far the most well-known aspect of microwave radiation is the cosmic microwave background (CMB), which has a near-perfect blackbody spectrum, of 2.73 K; this peaks at around 1.9 mm (160 GHz – the peak differs when measured by wavelength, from when measured by frequency).

The workhorse detector, in microwave astronomy (and much of radio astronomy, in general), is the radiometer, whose operation is described in considerable detail on this NRAO (National Radio Astronomy Observatory) webpage. The particular kind of radiometer which Penzias and Wilson used in their discovery of the CMB (at 7.35 cm, well away from the CMB peak) was a Dicke radiometer, designed by Robert Dicke (to search for the CMB!). And it was six differential microwave radiometers aboard the Cosmic Background Explorer (COBE) which first detected the CMB anisotropy, firmly establishing the CMB as the highly redshifted surface of last scattering (when baryonic matter and photons decoupled).

The microwave region, especially the short (millimeter) wavelength end, is a rich region for astrophysics, allowing the study of galaxy formation and evolution, stellar and planetary system birth, the composition of solar system body atmospheres, in addition to the CMB. There are already several observatories – many consortia – active in these fields; for example CARMA (Combined Array for Research in Millimeter-wave Astronomy), and ALMA (Atacama Large Millimeter/submillimeter Array) … astronomers just LOVE acronyms! (and no, that is not an acronym).

A new kind of microwave astronomical observatory has recently begun making obserations, the Allen Telescope Array, which provides instantaneous frequency coverage from 500 MHz to 11 GHz (among many other firsts). In many ways this serves as a technology demonstrator for the much more ambitious Square Kilometre Array.

Some of the many Universe Today stories on microwave astronomy are Probing the Large Scale Structure of the Universe, Dark Matter Annihilation at the Centre of the Milky Way, and Oldest and Most Distant Water in the Universe Detected.

Between them, Astronomy Cast episodes Radio Astronomy and Submillimeter Astronomy do a nice job of explaining microwave astronomy!